What is a link worth with regard to search engine rankings, and what kind of links should you hunt for if you're not the WSJ?

The most valuable links are hard to get. That does not mean that it is impossible for an eCommerce site or a site providing niche services to get well linked, and to get ranked by search engines based on those inbound links .

If you can submit your URL to a Web site and you get linked automatically, that's a useless link, regardless of the PageRank your Google Toolbar indicates for the source page. If you can drop your links in a forum's signature lines and posts, that's in most cases a powerless link, regardless whether the forum software adds a link condom or not.

Does that mean you should not spread low valued links? No, just don't rely on cheap links. URL drops and a handful of directory submissions can and should be part of your natural mixture of incoming linkage.

Does that mean you shouldn't drop your links in forums, user groups, blogs and alike? No, it means you shouldn't bother with me-too posts, enhance your links instead. Write an outstanding post, and link to your deep pages and other resources as well to point to related information. An informative and well thought out post will attract replies, so the forum thread gets more content, and ranks higher in the thread list caused by many views and replies. Perhaps the thread gets even linked from pages outside the forum's domain.

Having links in a popular thread helps you in two ways. First, as more links point to the thread, as more reputation (link popularity, PageRank, topic relevancy, TrustRank...) the thread will pass to your pages. Reputation is the underlying load of a link from a search engine's perspective, and the engines have developed a couple of ranking formulas to measure the reputation a link passes to the destination page.

The PageRank formula for example looks at the score of the source page, divides the source page's PageRank by the number of outgoing links, decreases the result by a dampening factor to take into account that a visitor of the source page will not follow every link, and passes the remaining portion of PageRank to the destination page. From this -- pretty much simplified! -- explanation it's obvious that PageRank is easy to manipulate by artificial linkage, but don't try it, you cannot outsmart Google on the long haul. The same goes for TrustRank and other ranking algorithms.

The above said leads to the second and more important reason why well placed deep links in a popular forum thread do help your site's SE rankings: they generate traffic to the landing pages. Forum visitors landing on your interior pages may bookmark your stuff, and come back, that's extremely valuable recurring traffic. A minority of those visitors may even link to your pages, for example from their community blogs or by adding your pages to a social bookmarking service like delicious or furl, or just by dropping your URL onto their own forum posts.

By clicking your links and surfing your site following navigational links, and by bookmarking your pages, a few of your visitors leave tracks in search engine databases, and the engines do use those in their rankings: popular pages do get a ranking boost. The engines capture such information via their toolbars and by spidering social bookmarking services (more information).

To summarize the above said, posting great content on forums generates human traffic during the life cycle of a thread, and it boosts your search engine rankings to some degree. Due to the nature of the beast, this works only when you constantly participate in public discussions, because even popular threads get buried in the archives sooner or later. Forum posts can build a good basis of inbound links, before trusted and more static links from authority sites come to the mix, or when the nature of your content makes it impossible to get links from authority sites like the W3C and high ranked hubs like the LII.

If you do it right, a search for your page titles and related terms will bring your URLs on the SERPs eventually. If you fail, and in the very beginning of your campaign, the forums will outrank you even for your domain name.

Keep this rule of thumb in mind: Valuable links generate human traffic. Even if only a handful of visitors land on your pages, the link is valuable. Don't bother with links placed where no human visitor clicks them. Although the search engines will discover most of these non-prominent links, and may even spider the destination page following them, unclicked links do not improve your search engine rankings, and they will not attract (new) visitors.

Besides active participation in communities like forums, there is another way to build a stable base of recurring visitors: establish your own community. You can add a blog to every type of Web site, even an e-Commerce site or a niche directory will profit from an integrated active blog. Blog interesting stuff frequently, tag all posts properly, ping all blog search engines, reply to comments in a timely manner, repeat.

Once you've produced a fair amount of good blog posts, go out and searchforrelatedblogposts. If you can add value to another blog post, write a comment and drop a link to one of your posts in a non-spammy way. In most cases those links don't pass reputation to your blog, but noisy bloggers will follow them and perhaps blog about your blog if they like it. Do not spam other blogger's comments, that is don't comment on each and every post, and don't always include a link to your blog in the message body. Politely build reputation, try to become an authority in your field, and you will be cited more and more, that is link love from the blogosphere floods in continuously.

Another way to create a user base is to interact with visitors, for example allow your visitors to post comments and product reviews, do weekly polls and so on, if it makes sense even add a forum. Be creative. Try new things frequently, and maintain working concepts actively. Search engines follow their users, so if you're able to generate traffic, you'll gain fair search engine placements naturally.